The present invention relates to systems and methods in the field of ionizing radiation imaging and more particularly to a system and method for digital detection of X-ray images.
There are described in the patent literature numerous systems and methods for the recording of X-ray images. Conventional X-ray imaging systems use an X-ray sensitive phosphor screen and a photosensitive film to form visible analog representations of modulated X-ray patterns. The phosphor screen absorbs X-ray radiation and emits visible light. The visible light exposes the photosensitive film to form a latent image of the X-ray pattern. The film is then chemically processed to transform the latent image into a visible analog representation of the X-ray pattern.
Recently, there have been proposed systems and methods for detection of static and or dynamic X-ray images. These digital X-ray systems and methods provide digital representations of X-ray images in which the X-ray image is recorded as readable electrical signals, thus obviating the need for films and screen in the imaging process. Digital X-ray systems typically rely on direct conversion of X-rays to charge carriers or alternatively indirect conversion in which X-rays are converted to light which is then converted to charge carriers.
Direct conversion approaches typically use an X-ray sensitive photoconductor such as amorphous selenium overlying a solid state element which comprises a solid state array having thin-film-transistors (TFT) or diodes coupled to an array of storage capacitors. An example of a direct conversion approach is provided by U.S. Pat. No. 5,313,066 to Lee et al., which describes an X-ray image capturing element comprising a panel having a layered structure including a conductive layer comprising a plurality of discrete accessible microplates and a plurality of access electrodes and electronic components built on the panel.
A further example of a direct conversion approach is U.S. Pat. No. 5,652,430 to Lee which describes a radiation detection panel made up of an assembly of radiation detector sensors arrayed in rows and columns where each sensor includes a radiation detector connected to a charge storage capacitor and a diode.
Indirect conversion approaches typically use a scintillating material such as columnar cesium iodide overlying a solid state active matrix array comprising photodiodes. The X-rays are converted to light by the scintillating material and the light is converted to charge by the photodiodes. An example of an indirect approach is provided by U.S. Pat. No. 5,668,375 to Petrick et al. which describes a large solid state X-ray detector having a plurality of cells arranged in rows and columns composed of photodiodes.
A further example of an indirect approach is provided by U.S. Pat. No. 5,801,385 to Endo et. al which describes an X-ray image detector having a plurality of photoelectric conversion elements on an insulating substrate.
Direct and indirect conversion based digital X-ray detectors use charge storage matrices to retain imaging information, which is then electronically addressed, with stored charge read out taking place subsequent to exposure. In dynamic imaging such as fluoroscopy, xe2x80x9creal-timexe2x80x9d images are simulated by repeatedly reading the integrated radiation values of the storage matrix to provide a sufficiently high number of frames per second, e.g. 30 frames per second. Image information, which is retained in the charge storage matrix, is not available until after the end of the X-ray pulse, since the detectors are operated in a storage mode. Thus, measurements made from the current generation of digital detectors are not real-time.
For medical diagnosis, it is desirable to use the minimum X-ray exposure dose that will provide an image having acceptable contrast and brightness.
The actual X-ray exposure dose for a specific X-ray examination may be selected using predetermined imaging exposure parameters and patient characteristics loaded from periodically updated lookup tables into a X-ray system console. Alternatively, the actual dose may be adjusted automatically using automatic exposure control devices, typically placed in front of the X-ray detector, to provide real-time control feedback to an X-ray source.
Automatic exposure control devices, which must operate in real-time, typically make use of a multi-field ion chamber or a segmented phototimer as described in U.S. Pat. No. 5,084,911. These devices sense radiation passing therethrough and provide a signal which terminates the X-ray exposure when a predetermined dose value, yielding a desired contrast level, has been reached.
Disadvantages of conventional exposure control devices include the fact that the real-time exposure signals are averaged over a fixed field area and do not directly correspond to the image information in a region of interest; the fact that devices located in front of the detector cause non-uniform attenuation of the X-rays and cause some of the radiation that would otherwise contribute to the signal at the detector to be lost; the fact that the devices are typically bulky and require external power sources; and the fact that the spectral sensitivity of the devices differs from that of the radiation image detector being used thus requiring corrections and calibrations for different X-ray tube voltage (kVp) values.
Efforts have been made to incorporate real-time exposure control into digital X-ray detectors, particularly those detectors based on the xe2x80x9cindirectxe2x80x9d conversion approach.
An example of apparatus for use in detecting real-time exposure information for an xe2x80x9cindirectxe2x80x9d scintillator based digital detector is described in U.S. Pat. No. 5,751,783 to Granfors et. al. This patent describes an exposure detection array of photodiodes positioned behind an imaging array of photodiodes. The exposure detection array, which is a separate component involving separate electronics, is used to detect light which passes through the imaging array in certain regions due to gaps between adjacent pixels caused by a relatively low pixel fill factor. Pixels are regionally grouped to provide regional density measurements.
Alternatively, for digital X-ray imaging, special methods have been proposed allowing digital detectors to sample the exposure prior to the imaging exposure using a two step method, thus simulating real-time exposure information. An example of a two-step exposure method is described in U.S. Pat. No. 5,608,775 to Hassler et al In that method exposure information for a digital detector is generated by first exposing the detector to a xe2x80x9ccalibratingxe2x80x9d pulse in which an X-ray exposure of short duration produces an exposure in a solid state detector, which is then processed to calculate the X-ray transparency of the body being imaged in order to determine an optimum X-ray dose.
There is thus provided in accordance with a preferred embodiment of the present invention, an ionizing radiation imaging sensor for providing integrated radiation information based on a new high resolution digital X-ray detector suitable for ionizing radiation imaging, and in particular X-ray imaging for general radiography diagnostics.
There is thus provided in accordance with a preferred embodiment of the present invention, an ionizing radiation image sensor having an ionizing radiation sensitive element, a generally pixellated array of capacitors cooperating with the ionizing radiation sensitive element, and a charge source which is operative to electrically charge the pixellated array of capacitors through a gap.
Preferably, each capacitor of said generally pixellated array includes an electrode having at least one conducting plate which is at least partially exposed for charge injection thereto.
In further accordance with a preferred embodiment of the present invention, there is provided an ionizing radiation image sensor having an ionizing radiation conversion multilayer element which is operative to convert impinging ionizing radiation to electrical charge, an external charge source which is operative to emit electrical charge; and an array of storage capacitors disposed between the ionizing radiation conversion multilayer element and the external charge source, the storage capacitors being operative to sink charge to or source charge from the ionizing radiation conversion multilayer element and to sink charge to or source charge from the external charge source.
Preferably, the ionizing radiation image sensor is sensitive to X-ray. Moreover, the ionizing radiation conversion multilayer element preferably includes at least one layer which directly converts X-ray radiation to electrical charge.
Further in accordance with a preferred embodiment of the present invention, the one layer which directly converts X-ray radiation to electrical charge is formed from amorphous selenium doped with at least one of arsenic and chlorine.
In accordance with one preferred embodiment of the present invention, the radiation conversion multilayer element includes at least one layer which converts X-ray radiation to optical radiation. This layer may be formed from one of the following materials: cesium iodide doped with thallium and cesium iodide doped with sodium.
There is also provided in accordance with another preferred embodiment an ionizing radiation image readout device having an ionizing radiation sensitive element which is operative to convert impinging X-ray radiation to an electrical charge image; and a storage capacitor array operative to store the electrical charge image. The storage capacitor array has a matrix array of plate electrodes; a linear array of elongate electrodes. Preferably the storage capacitor array is addressed via the plate electrodes and a charge image readout is carried out via the elongate electrodes.
Preferably, the ionizing radiation image readout device includes at least one charge source which addresses the storage capacitor array by charge injection in a row-by-row manner. The charge injection preferably results in generally uniform charging of said matrix array of plate electrodes.
There is also provided in accordance with yet another preferred embodiment of the present invention, an ionizing radiation imager including a first array of storage capacitors which stores a charge pattern representing an ionizing radiation image at a first resolution; a second array of storage capacitors, capacitively coupled to the first array of storage capacitors, which stores a charge pattern representing the ionizing radiation image at a second resolution; integrated radiation data readout electronics connected to the first array of storage capacitors; and realtime radiation data readout electronics connected to the second array of storage capacitors.
Preferably, the integrated radiation data is taken at a generally high resolution and the real-time radiation data is taken at a generally lower image resolution. In accordance with one embodiment of the present invention, the integrated radiation data readout electronics provides data representing an X-ray image and the real-time radiation data provides feedback for automatic exposure control.
There is also provided in accordance with another preferred embodiment of the present invention a method for ionizing radiation imaging which includes providing an ionizing radiation sensitive element including an array of storage capacitors coupled thereto, charging the array of storage capacitors to a generally uniform voltage level using a non-contact proximity charge source; exposing the ionizing radiation sensitive element to impinging ionizing radiation causing imagewise discharge of the charged array of storage capacitors thus creating an electrical charge pattern therein corresponding to an ionizing radiation image; and charging said the of storage capacitors to a generally uniform voltage level using a non-contact proximity charge source which causes readout of the electrical charge pattern.
In accordance to one embodiment of the present invention the charging includes charge injection to each capacitor of said array via at least one conducting plate which is at least partially exposed for charge injection thereto.
There is also provided a method for ionizing radiation imaging including providing an ionizing radiation conversion multilayer element which converts impinging ionizing radiation to electrical charge; causing an external charge source to emit electrical charge; and causing an array of storage capacitors disposed between the ionizing radiation conversion multilayer element and the external charge source to sink charge to or source charge from the ionizing radiation conversion multilayer element and to sink charge to or source charge from the external charge source.
In accordance with one embodiment of the present invention, the ionizing radiation conversion multilayer element is sensitive to X-ray. The ionizing radiation conversion multilayer element may typically include at least one layer which directly converts X-ray radiation to electrical charge.
In accordance with an alternative embodiment of the present invention, the ionizing radiation conversion multilayer element includes at least one layer which converts X-ray radiation to optical radiation.
There is also provided in further accordance with an embodiment of the present invention, an ionizing radiation image readout method which includes causing an ionizing radiation sensitive element to convert impinging X-ray radiation to an electrical charge image; and storing the electrical charge image on a storage capacitor array including a matrix array of plate electrodes and a linear array of elongate electrodes; addressing the storage capacitor array via the plate electrodes; and reading out the electrical charge image via the elongate electrodes.
There is also provided in yet further accordance with an embodiment of the present invention, a method for ionizing radiation imaging including the causing of a first array of storage capacitors to store a charge pattern representing an ionizing radiation image at a first resolution; the causing of a second array of storage capacitors, capacitively coupled to the first array of storage capacitors, to store a charge pattern representing the ionizing radiation image at a second resolution; reading out real-time radiation image data from the second array of storage capacitors; and reading out integrated radiation image data from said first array of storage capacitors.
This method may also include the step of effecting real-time exposure control employing said real-time radiation image data.